1. Field of the Invention
The present invention relates to a communication method used in a mobile communication system, and more specifically to a handover control performed in a mobile communication system.
2. Description of the Related Art
In the mobile communication system such as a mobile telephone system, etc., there can be a bit error occurring in a wireless region (that is, a wireless link between a mobile terminal and a base station depending on the position of a mobile terminal or the radio wave circumstance between the mobile terminal and the base station. Therefore, a system for implementing a retransmission control function for retransmitting data when an error occurs in a wireless region has conventionally been known. For example, high-speed downlink packet access (HSDPA) and high-speed uplink packet access (HSUPA) standardized in the third generation partnership project (3GPP) have the function of performing retransmission control between a base station and a mobile terminal when a bit error occurs in a wireless region.
However, when a mobile terminal moves from a communication area of a base station to a communication area of another base station, handover is performed to continue the communication. When the handover is performed, the radio waves transmitted from the mobile terminal reaches a plurality of base stations. However, in the HSDPA, the so-called hard handover is performed. That is, in the hard handover, a mobile terminal is not simultaneously connected to a plurality of base stations. On the other hand, in the HSUPS, a mobile terminal can be simultaneously connected to a plurality of base stations. In this case, an upper level device selects and combines data. That is, the data transmitted from a mobile terminal is received by a plurality of base stations, and the upper level device selects one of them. However, since a circuit from a base station to the upper level device transmits information not to be used, a wasteful configuration is designed. Especially when there is a large amount of data, an influence on the system capacity becomes large. When the hard handover is performed in the system corresponding to the HSUPA, the first base station performs retransmission control if a transmission error occurs before the handover from the first base station to the second base station, and the second base station performs the retransmission control if a transmission error occurs after the handover.
The patent document 1 (Japanese Published Patent Application No. H10-145835) describes a mobile communication system in which a plurality set of frames including the same data are transmitted from a mobile terminal to an exchange apparatus via different base stations while a handover is performed in the mobile communication system, and the frame having the lowest error rate is selected by the exchange apparatus and transferred to a destination. In this system, the mobile terminal is simultaneously connected to a plurality of base stations while the handover is performed. That is, in this system, so-called software handover is performed. However, the patent document 1 does not practically describe the retransmission control performed when a transmission error occurs.
In the mobile communication system in which the hard handover is performed, when the radio environment between the mobile terminal and the base stations is undesired, the data retransmission frequency between them becomes high, and the data transmission efficiency becomes low.
In the system described in the patent document 1, since data is constantly transmitted from a plurality of base stations to an exchange apparatus while the handover is performed, the load of a network between them is heavy.